PLT OF A MOONEY M20C CLBED ABOVE ASSIGNED ALT DURING IFR CRUISE DUE TO ATTEMPTING TO STAY OUT OF CLOUDS RESULTING IN ARTCC RADAR CTLR INTERVENTION.

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RECEIVED A STANDARD BRIEFING FROM FLT SVC AND FILED AN IFR PLAN FROM W91 (MONETA; VA) TO FDK (FREDERICK; MD). WHILE FLYING (MOONEY M20C); AFTER ABOUT 10 MINS ENTERED IMC CONDITIONS (CLOUDS). MY ASSIGNED ALT WAS 7000 FT. AFTER APPROX 20 MINS IN SOLID CLOUDS; REQUESTED AND RECEIVED 9000 FT. AFTER APPROX 5-10 MINS I EXITED THE CLOUDS AND IMMEDIATELY OBSERVED A VERY DARK GRAY CELL DIRECTLY IN FRONT WHICH I INSTINCTIVELY KNEW THAT I DID NOT WANT TO PENETRATE AND THAT I WOULD JUST SKIM THE TOP AT MY EXISTING/ASSIGNED ALT OF 9000 FT. JUST AS I WAS ABOUT TO ENTER THE VERY TOP OF THE DARK GRAY CELL; ATC CALLED AND ASKED ME MY ALT AND I PROMPTLY REPLIED TELLING HIM THE TRUTH THAT I WAS AT 9300 FT INADVERTENTLY WHILE TRYING TO SKIM THE TOP OF THE CELL AND THAT I WAS IMMEDIATELY CORRECTING TO 9000 FT AFTER REQUESTING A CURRENT ALTIMETER SETTING. WHILE DSNDING INTO THE CELL AND LEVELING OFF AT 9000 FT THE CTLR GAVE ME A MILD REPRIMAND SAYING THAT IF EVERYONE DID THAT HE WOULD NOT BE ABLE TO MAINTAIN THE REQUIRED 1000 FT SEPARATION. AFTER I RECEIVED AN UPDATED WX RPT FROM FLT WATCH FOR FDK; I REQUESTED AN AMENDMENT TO MY FLT PLAN AND REQUESTED A 180 DEG TURN TO RETURN TO LYH (LYNCHBURG) AND A FINAL IFR APCH INTO W91 (MONETA; VA) I EXECUTED A VOR DME RWY 23 APCH INTO W91. END OF FLT.